A health warning was sounded last night over the dangers of eating soya after two senior American government scientists revealed that chemicals in the product could increase the risk of breast cancer in women, brain damage in men and abnormalities in infants.

The disclosure, which sent shockwaves through the multi-billion dollar food industry, came after the scientists decided to break ranks with colleagues in the US Food and Drug Administration and oppose its decision last year to approve a health claim that soya reduced the risk of heart disease. They wrote an internal protest letter warning of 28 studies revealing toxic effects of soya.

In an interview with The Observer, one of the scientists, Daniel Doerge, an expert on soya, said: 'We have very real worries that this health claim will be used by the industry as an endorsement of much wider health benefits to soya beyond the heart. Research has shown a clear link between soya and the potential for adverse effects in humans.'

BSE and other health scares related to meat have led to rocketing sales of soya-related products in Britain. But it is not just vegetarian foods such as tofu that use soya. It is a key ingredient in products from meat sausages and fish fingers to salad creams and breakfast cereals.

The concerns of Doerge and fellow FDA researcher Daniel Sheehan focus on chemicals in soya known as isoflavones which have effects similar to the female hormone oestrogen.

While these chemicals may help to prevent a range of conditions including high cholesterol, they also lead to health problems in animals including altering sexual development of foetuses and causing thyroid disorders. Although soy is thought to protect against breast cancer, some studies show that chemicals in soya may increase the chances of breast cancer which uses oestrogen-type hormones for growth.

Their letter to the FDA seen by The Observer states: 'There is abundant evidence that some of the isoflavones found in soy demonstrate toxicity in oestrogen sensitive tissues and in the thyroid. Additionally, the adverse effects in humans occur in several tissues.

'During pregnancy in humans, isoflavones per se could be a risk factor for abnormal brain and reproductive tract development.'

This will frighten mothers who increasingly use soya milk for babies. Doerge said: 'They are exposing their children to chemicals which we know have adverse effects in animals. It's like doing a large uncontrolled and unmonitored experiment on infants.'

The soya industry insists that most research shows the health benefits of soya outweigh risks and that adverse effects seen in animals do not apply to humans.

Richard Barnes, European director of the US Soy Bean Association, said: 'Millions of people around the world have been eating soya for years and have shown no signs of abnormalities or disorders.'